The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is known. One known process, more commonly known as xerography, involves the formation of an electrostatic latent image on the surface of a photosensitive plate which is usually referred to as the photoreceptor. The photoreceptor itself contains a conductive substrate containing on its surface a layer of photoconductive insulating material; and in many instances there can be used a thin barrier layer between the substrate and the photoconductive layer to prevent charge injection from the substrate into the photoconductive layer upon charging of the plate surface since if charge injection were allowed this would adversely affect the quality of the resulting image. This process involves forming an electrostatic latent image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to an imagewise pattern of activating electromagnetic radiation. The light-struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge is selectively dissipated in these irradiated areas. After the photoconductor is exposed, the electrostatic latent image on this image-bearing surface is typically rendered visible with a finely divided colored marking material, known in the art as “toner”. This toner will be principally attracted to those areas on the image-bearing surface, which retain the electrostatic charge and thus form a visible powder image. The electrostatic latent image may also be used in a host of other ways as, for example, electrostatic scanning systems may be employed to “read” the latent image or the latent image may be transferred to other materials by Transfer of ElectroStatic Images (TESI) techniques and stored. A developed image can be read or permanently affixed to the photoconductor where the imaging layer is not to be reused.
In the commercial “plain paper” copying systems, the latent image is typically developed on the surface of a reusable photoreceptor, subsequently transferred to a sheet of paper and then permanently affixed thereto to form a permanent reproduction of the original object. The imaging surface of the photoreceptor is then cleaned of any residual toner and additional reproductions of the same or other original objects can be made thereon.
Various types of photoreceptors are known for use in electrophotographic copying machines. For example, there are known in the art photoreceptors wherein the charge carrier generation and charge carrier transport functions are performed by discrete contiguous layers. There are also known in the art photoreceptors, which include an overcoating layer of an electrically insulating polymeric material. In conjunction with such so-called “overcoated” photoreceptors there have been proposed a number of imaging methods. Nevertheless, as the art of xerography advances and more stringent demands are imposed upon the carrying apparatus because of increased performance standards there continue to be discovered novel imaging methods. The present application relates to a novel electrophotographic imaging method which utilizes an overcoated electrophotographic imaging member.